The present invention relates to an optical wavelength multiplex transmission system using repeaters and, particularly, to a transmission system in which a plurality of wavelength-multiplexed signal lights are transmitted through an optical transmission line having a plurality of optical amplifying repeaters.
The optical wavelength multiplex transmission system using the repeaters includes basically an optical transmitter unit, an optical receiver unit and an optical transmission line connecting the optical transmitter unit to the optical receiver unit. The optical transmitter unit includes a first to n-th light sources, a first to n-th optical modulators corresponding to the respective first to n-th light sources, a first to n-th optical amplifiers, an optical coupler for multiplexing optical signals from the respective optical amplifiers and a boosting optical amplifier, where n is an integer larger than 1. The optical receiver unit includes a light distributer, a band-pass filter and an optical receiver for receiving the respective signal lights. The optical transmission line includes usually a plurality of optical amplifying repeaters.
In the optical transmission unit, the light sources emit optical signals having different first to n-th wavelengths, respectively. These optical signals are supplied to the respective optical modulators. Modulated optical signals from the optical modulators are amplified by the optical amplifiers, respectively, wavelength-multiplexed by the optical coupler and supplied to the boosting optical amplifier. The boosting optical amplifier amplifies the wavelength-multiplexed optical signal and a resultant optical wavelength multiplex signal is transmitted from the optical transmitter unit as an optical transmitting signal, relayed by the plurality of the optical amplifying repeaters of the optical transmission line and supplied to the optical receiver unit.
The optical receiver unit receives the optical transmitting signal as an optical receiving signal, the optical distributor branches the optical receiving signal to a first to n-th optical signals. The branched optical signals are supplied to the optical band-pass filters, respectively. Each of the optical band-pass filters allows only one of the optical signals which has a predetermined wavelength assigned thereto to pass through. The optical signals passed through the band-pass filters are supplied to the first to n-th optical receivers, respectively.
In the optical wavelength multiplex transmission system which uses the repeaters and has the construction as mentioned above, optical S/N ratios of the transmitting optical signal depend upon signal levels of optical wavelength components of the optical signal after transmission and optical noise, that is, amplified spontaneous emission (referred to as "ASE", hereinafter) generated in the optical amplifying repeaters in the transmission line. However, because of the gain-wavelength characteristics of the optical amplifying repeater, the larger the attenuation of optical wavelength component of the wavelength multiplexed optical signal is the remoter the wavelength thereof from a center wavelength of the multiplexed optical signal. As a result, there is a large deviation of optical S/N ratio produced between the optical wavelength components of the optical receiving signal after transmission. When there is such large deviation of optical S/N ratio between the respective optical wavelength components, the optical receiver unit can not receive the multiplexed optical transmission signal exactly.
An example of a method for removing such deviation of optical S/N ratio, that is, equalizing the optical S/N ratios, is disclosed in, for example, Neal S. Bergano, et al., "40 Gb/s WDM Transmission of Eight 5 Gb/s Data Channels Over Transoceanic Distances using the Conventional NRZ Modulation Format", OFC'95 Post Dead Line, PD19-1 to PD19-5, Feb. 24, 1995. According to the disclosed method, after an optical transmission line having a plurality of optical repeaters is built, optical spectrum is measured at a receiving end of the optical transmission line. On the basis of a result of the measurement, signal levels of optical wavelength components of the respectively transmitting optical signals are regulated in an optical transmitter unit to equalize optical S/N ratios of the optical wavelength components of the receiving optical signal. This procedures are repeated until the optical S/N ratios of the wavelength components of the receiving optical signal becomes exactly the same.
However, the above mentioned conventional equalizing method has some problems. A first one of the problems is that, after the optical transmission line is built, it is necessary to regulate the levels of the transmitting optical signals in the optical transmitter unit by communicating between the opposite ends of the transmission line, which is very troublesome. A second problem is that, since a regulation range of the optical levels of the respective wavelength components of the optical signal in the optical transmitter unit is known only after the construction of the transmission line, it is necessary to preliminarily prepare a very wide regulation range of optical signal level in the optical transmitter unit.